The present invention relates to electrodes for capacitors and other electrical applications and specifically to fabricating field-graded electrodes and to methods of adhering "diamondlike carbon" (DLC) films to electrodes and other substrates.
Capacitors are commonly employed as elements in electrical circuits to reduce voltage fluctuations in electronic power supplies, to transmit pulsed signals, to generate or detect electromagnetic oscillations at radio frequencies, and to provide electronic time delays. The simplest capacitor consists of two plane, parallel plates of an area, A, separated by a distance, d. If a voltage, V, is applied to the plates, a +q charge will develop on one plate and a corresponding -q charge will develop on the other plate. The charge q developed is related to the applied voltage V by a proportionality constant, C, called the capacitance of the capacitor. The effect of filling the space between the electrodes with a dielectric is to increase the capacitance by a factor, .kappa., called the dielectric constant. Filling the space between the electrodes with a dielectric also increases the charge density, on each plate in proportion to the dielectric constant .kappa..
DLC films are characterized by an amorphous, polymer-like hydrogen and carbon structure but exhibit physical properties similar to those of single-crystal diamond. Researchers have recognized that the mechanical and electrical insulating properties of DLC films make them, like diamonds, attractive for use in electronic applications.
Thin DLC films suitable for electrical applications have been deposited using a variety of techniques such as plasma enhanced chemical vapor deposition (PECVD). The PECVD technique involves the imposition of a plasma discharge generated by, for example, a capacitively or inductively coupled RF, microwave or DC discharge.
Improvements in and especially closer control of the electrical properties of DLCs would be well received by the electrical industries. Moreover, although conventional DLC films have breakdown voltages exceeding 1 MV/cm, DLC films with improved breakdown strengths also would be well received by the electrical industries. For example, the energy density or storage in a capacitor is directly proportional to the square of its operating stress as determined by the breakdown voltage of its dielectric medium and as given by the equation: EQU .tau.=.kappa..epsilon..sub.0 .rho. (1)
where D is the energy density in the capacitor, .kappa. is the dielectric constant of the dielectric medium, .epsilon..sub.0 is the permittivity of free space, and E is the applied electric field. That is, energy storage is maximized in capacitors having dielectrics with relatively high breakdown voltages. Exceeding the breakdown voltage of the dielectric, however, results in premature capacitor failure. DLC films having improved breakdown voltages therefore are desirable in the production of capacitors having correspondingly higher energy densities.
To be useful as capacitor dielectrics, DLC films must not only possess relatively high resistivities and energy densities, but must also adhere tenaciously to metal (e.g., aluminum) electrodes without spalling. Aluminum, for example, is the metal of choice for capacitor electrodes because of its inherently low density. For high voltage applications however, the DLC film thicknesses required to support high voltages are characterized by large internal stresses. These internal stresses often cause delaminations in aluminum-DLC-aluminum composite structures. Although pre-cleaning of the substrate surface with an argon plasma etch may improve adherence, Gehan et al., "influence of DC Bias Voltage on the Refractive Index and Stress of Carbon-Diamond Films Deposited From a CH.sub.4 /Ar Plasma", J. Appl. Phys., Vol. 70 (10) (Nov. 15, 1991), the disclosure of which is expressly incorporated herein by reference, have reported that pre-cleaning must be carefully performed as it has the potential to cause significant damage to the surface of the substrate. Such surface damage is magnified in capacitor applications inasmuch as any defect on the electrode will result in an electric field enhancement leading to an increased charge injection and a corresponding lowering of the breakdown voltage of the dielectric and the energy storing capacity of the capacitor. Accordingly, methods for depositing DLC films onto, for example, smooth aluminum electrodes and for fabricating field-graded electrodes prior to, for example, the deposition of DLC films thereon would be well received by the electrical industries.